1. Field of the Invention
This invention relates broadly to a system and method for compositional analysis of generally hydrophobic products (such as lubricants, edible oils, and fuels) using infrared spectroscopy. More particularly, the invention relates to systems and methods for determining the total acid content of such products using Fourier Transform Infrared (FTIR) spectroscopy.
2. State of the Art
Infrared (IR) spectroscopy is the subset of spectroscopy that deals with the infrared region (e.g., typically including wavelengths from 0.78 to approximately 300 microns) of the electromagnetic spectrum. It covers a range of techniques, the most common being a form of absorption spectroscopy. As with all spectroscopic techniques, it can be used to identify compounds or investigate sample composition. A common laboratory instrument that uses this technique is an infrared spectrophotometer. Infrared spectroscopy exploits the fact that molecules have discrete rotational and vibrational energy levels and absorb infrared light at specific frequencies that are determined by the differences in energy between these discrete energy levels.
In IR absorption spectroscopy, the infrared spectrum of a sample is recorded by passing a beam of infrared light through the sample or placing the sample on the surface of an internal reflection element through which a beam of infrared light is passed by total internal reflection. Measurement of the transmitted or totally internally reflected light striking a detector reveals how much energy was absorbed at each wavelength. This can be done with a monochromatic beam, which changes in wavelength over time. Alternatively, a polychromatic IR beam (e.g., a range of IR wavelengths) can be passed through the sample to measure a range of wavelengths at once. From this, a transmittance or absorbance spectrum can be produced, showing the IR wavelengths at which the sample absorbs. Analysis of these absorption characteristics reveals details about the molecular structure of the sample.
Fourier Transform Infrared (FTIR) spectroscopy is a form of IR absorption spectroscopy that utilizes an interferometer placed between a polychromatic source of IR light and the sample. Measurement of the light striking the detector produces an interferogram. Performing a Fourier transform on the interferogram shows the IR wavelengths at which the sample absorbs. The development of FTIR technology has substantially enhanced the utility and sensitivity of IR spectroscopy as a tool for quantitative analysis. In addition, various data analysis techniques have been developed to facilitate accurate quantitative analysis of highly complex sample mixtures subjected to IR spectroscopic examination. The information inherent in the infrared spectrum of such sample mixtures includes information at the molecular level about the chemical composition of the mixture. Thus, FTIR technology and analysis allows for the determination of the concentrations of the components in the sample mixture, and for the detection of contaminants or other unwanted chemical components or compounds in the sample mixture.
One area in which FTIR spectroscopy has been extensively utilized is in the monitoring of the condition of lubricating fluids, an activity which has commonly been performed in commercial laboratories. For example, FTIR spectroscopy has been employed to monitor the levels of additives present in such fluids and of degradation products that may be generated as a result of breakdown of the fluid. In another example, the amount of water present in lubricating oils has been quantitated by means of a “splitting” method that utilizes a stoichiometric reaction between water and 2,2-dimethoxypropane (DMP) to produce acetone, a product which is readily measured by IR spectroscopy. This splitting method includes diluting an oil sample and splitting it into two parts. One of the two parts is treated with a blank reagent. The other part is treated with a reactive reagent (DMP). Both parts are then analyzed using FTIR spectroscopy, and moisture measurements are obtained by subtracting the spectrum of the sample treated with the blank reagent from the spectrum of the sample treated with the reactive reagent to eliminate the spectral features of the oil, leaving only the spectral changes related to the reaction. This FTIR method was an improvement over the ASTM Karl Fischer (KF) titration method, a methodology commonly used to measure water in oil samples. It allowed the amount of moisture in an oil sample to be quantified while avoiding the limitations of the KF method, such as its susceptibility to oil additive interferences that affect the accuracy and precision of the data obtained.
Another area in which FTIR spectroscopic analysis has found application is in the analysis of edible fats and oils employed in the food industry, which are extracted from a wide range of raw materials and then refined and processed in various ways, for example, by fractionation and hydrogenation. In “Automated FTIR Analysis of Free Fatty Acids or Moisture in Edible Oils” (McGill University, Quebec Canada, JALA February 2006), a methodology for analyzing the free fatty acid content of edible oil mixtures is disclosed. The method utilizes methanol as the reagent solvent (which is completely immiscible with the edible oil) and employs NaHN—C≡N as a reagent to convert free fatty acids into methanol-soluble salts without causing saponification of the oils. This method includes thoroughly mixing an edible oil sample with the reagent solvent, allowing the mixture to stand to ensure separation of the oil and solvent layers, and then analyzing the free fatty acid content using FTIR analysis. As the edible oils being analyzed in this process are completely immiscible with the reagent solvent (methanol), interferences due to absorption of the IR light by the oil portion of the sample do not pose an issue in the analysis.